The employment of microprocessors and step motors for automating and controlling the pipetting process has greatly enhanced the convenience of pipetting. In a typical automated pipetting apparatus, the step motor is connected to piston pump. The piston within the piston pump is driven by the step motor. The displacement of the piston within the piston pump is proportional to the number of steps executed by the step motor. When the piston pump is pneumatically connected to a pipette, displacements of the piston can be employed to aspirate and express liquids therefrom. To a first approximation, the volume of liquid which is aspirated or expressed into or out of the pipette is directly proportional to the displacement of the piston and to the number of steps executed by the step motor.
However, the relationship between the volume of liquid which is aspirated or expressed is not strictly equal to the displacement of the piston and to the number of steps executed by the step motor. Inequality arises from the expansion of the air within the pipette due to weight of the liquid column within the pipette and the resultant reduction of air pressure therein. Mezei et al (U.S. Pat. No. 4,586,546) discloses the use of a microprocessor within a pipetting apparatus to compensate for the inequality caused by the reduction of air pressure within the pipette due to the weight of the liquid column.
Accuracy and precision were further improved by the introduction of a back sip function (e.g., IQ 190 DS Sample Processor, manufactured by Cavro Scientific Instruments Inc., Sunnyvale, Calif.). A back sip is executed after the expression of liquid from the pipette and causes liquid to withdraw into the pipette. After the execution of each piston stroke for expressing fluid, the microprocessor instructs the step motor to reverse direction and to displace the piston over a small volume in the opposite direction. This causes liquid to withdraw into the pipette. Hence, the back sip function reduces the occurance of unintended dripping from the pipette. Methods for programming a computer for executing a back sip funcion are described in the publication entitled Cavro RS232C Primitive Protocol Manual (August, 1984, P/N 015-5864 Rev. B, Cavro Scientific Instruments, Inc., Sunnyvale, Calif.).
Unfortunately, under some circumstances, the prior art back sip function does not reduce unintended dripping to the degree that would be anticipated. Furthermore, under other circumstances, the prior art back sip function effectively prevents unintended dripping but degrades the accuracy and precision of pipetting due to other factors. The prior art back sip is optimized by determining the best compromise for the piston displacement which performs a fixed back sip function, i.e. what single magnitude of piston displacement most effectively reduces unintended dripping and best improves the accuracy and precision of the pipetting process both when the pipette is near full and when it is near empty. The optimal piston displacement for the prior art back sip depends upon the gearing of the step motor, the diameter of the piston, and upon the configuration of the pipette. The magnitude of the optimal piston displacement is then converted to the number of step counts which correspond to such displacement. The step count is then executed by the step motor so as to cause the same optimal piston displacement for each back sip.
What was needed was both a recognition of the specific factors and causes of the poor performance of the prior art back sip function and a remedy for this problem.